The design and operation of single-loop, countercurrent spray towers utilizing limestone is discussed by Rader and Bakke, in Incorporating Full-Scale Experience Into Advanced Limestone Wet FGD Designs, presented at the IGCI* Forum 91, Sep. 12, 1991, Washington, D.C. (*formerly the Industrial Gas Cleaning Institute, now the Institute of Clean Air Companies, Washington, DC) Open spray towers (i.e., those not having packings, trays or other means for facilitating gas-liquid contact) are simple in design and provide high reliability.
The use of a variety of reagents has been suggested, but the most preferred are those which are effective without high additive levels and can be purchased at low cost and stored and transported with minimal special handling. Calcium carbonate (commercially available in a number of forms including limestone) is a material of choice because it meets these criteria and, when properly processed, yields process byproducts that can be easily disposed of as landfill or sold as gypsum.
In single-loop, countercurrent, open scrubbing towers of the type discussed by Rader and Bakke, a scrubbing liquid based on calcium carbonate flows downwardly while the SO.sub.x -laden effluent flows upwardly. They summarize historical values for a range of parameters, including absorber gas velocity (giving a minimum of 6 and a maximum of 15 feet per second, i.e. about 2 to less than 5 meters per second), indicating that absorber gas velocity has a weak influence on the liquid-to-gas ratio (L/G), a key factor in both capital and operating expenses. The height of the spray contacting zone in these towers is not given, but typical values will be on the order of from about 6 to about 15 meters, historically considered an important factor in engineering an efficient system which can be expected to reliably remove at least 95% of the SO.sub.x from combustion effluents.
In conventional towers of this type, the ratio of the quantity of slurry to the quantity of gas (L/G) is said to be arguably the single most significant design parameter. The L/G affects the cost of pumping, the cost of holding tanks and other operational and economic factors. The cost of pumping the limestone slurry increases proportionally with the tower height. It would be desirable to decrease L/G requirements and height for open spray towers.
Sulfur oxides (SO.sub.x), principally SO.sub.2, are absorbed in the descending scrubbing slurry and collected in a reaction tank where solid calcium sulfite and solid calcium sulfate are formed. Desirably, the reaction tank is oxygenated to force the production of the sulfate. Once the crystals of sulfate are grown to a sufficient size, they are separated from the slurry in the reaction tank.
In a paper by K. R. Hegemann, et al, entitled THE BISCHOFF FLUE GAS DESULFURIZATION PROCESS (presented at the EPA and EPRI cosponsored First Combined FGD and Dry SO.sub.2 Control Symposium, Oct. 25-28, 1988) a scrubbing tower is depicted as including a hydrocyclone loop which separates a gypsum slurry from a wet scrubber into a coarse solids stream and a fine solids stream, with the fine solids stream being returned to the scrubber. In U.S. Pat. No. 5,215,672, Rogers, et al. describe a process similar to that of Hegemann, et al. in that it employs a hydrocyclone as a primary dewatering device. In the latter case, after separating a fine solids stream from a coarse solids stream rich in gypsum, water as part of a thickened fines stream is disposed of along with at least a portion of the fines removed. Neither of the descriptions of these approaches, however, indicates how the use of a hydrocyclone as a primary dewatering device can be employed to improve overall process efficiency with a correspondingly higher process economy while decreasing the overall size requirements of the tower, improving reagent utilization, maintaining high reliability, reducing energy consumption, and achieving high throughputs with high percentage SO.sub.x reduction.
The art has also provided packed towers. Rader and Bakke point out that while these types of towers have some advantage in terms of decreased operating costs, they present additional risks. The packings or other gas-liquid mixing means can become clogged or corroded and cause unacceptable bypass or pressure drop, resulting in prolonged periods of downtime. It would be advantageous to have an open tower which had the advantages of the packed towers, but which did not require the packings, and was smaller than open towers of conventional construction.
The prior art does not directly address the points necessary to achieve improvements that, in the context of single-loop, open-tower, countercurrent limestone wet scrubbers for SO.sub.x reduction, permit results comparable to achieved with packed towers but without the use of packings or the problems associated with them.